When trying to cross build proteinortho using sbuild, the result is a
150 minute build timeout via inactivity. What actually happens is that
after linking proteinortho_clustering, the build system runs it and as
that fails proceeds to silently rebuilding lapack. Given that lapack is
non-trivial to compile and all of its output is hidden, this times out
the build.

We cannot expect to run this test during cross compilation. I therefore
propose to make the build system aware of cross compilation. Then, it
trusts that a successful link is ok. Once doing so, proteinortho quickly
cross builds successfully saving a lot of QA resources. The failure mode
has wasted around 9 CPU days so far. Please consider applying the
attached patch.

Helmut